Magnetic resonance imaging is a complex interaction between protons in biological tissues, a static and alternating magnetic field (the magnet), and energy in the form of radio-frequency waves of a specific frequency (RF), introduced by coils placed next to the subject. The energy state of the hydrogen protons is transiently increased. The subsequent return to equilibrium (relaxation) of the protons results in the release of RF energy which can be measured by the same surface coils that delivered the RF pulses. The RF energy, also referred to as the RF signal or echo, is complex and is thus transformed by Fourier analysis into useful information used to form an MR image.